1. Field of the Invention
The present invention relates generally to the field of polymer coated, implantable medical devices. More particularly, it concerns such medical devices comprising a first layer of a metal bonded to a short chain silicone terminating with silicon hydride, and a second layer of a silicone polymer covalently bonded to the silicon hydride.
2. Description of Related Art
It has become common to treat a variety of medical conditions by implanting medical devices into the esophagus, trachea, colon, biliary tract, urinary tract, vascular system, or other location in the body of a human or animal patient. For example, in the vascular system, valves in the heart may weaken as a result of age, injury, or disease, impairing proper blood flow and posing a health risk to the patient. Such weakened naturally-occurring heart valves may be replaced with prosthetic heart valves.
Well-known in the art are prosthetic heart valves which comprise a polymer stent to give the valve shape, structure, and rigidity, over which is sewn tissue taken from an animal heart valve. Such prosthetic heart valves have a number of shortcomings. First, the introduction of animal tissue into another species raises the risk of adverse body responses, such as rapid thrombogenic reaction and inflammatory tissue reactions. Second, the animal tissue is mechanically connected to the stent, and thus the heart valve has low shear resistance.
Attempts have been made to construct heart valves from different materials and adhere their components in different ways. For example, metal stents have been used and animal tissue sewn over them. In another example, polymers have been used in place of animal tissue over metal stents. Polymeric materials avoid the risk of immunogenicity posed by use of animal tissue, but heart valves comprising polymeric materials covering metal stents still suffer from low shear resistance.
In order to increase the shear resistance of heart valves comprising polymeric materials covering metal stents, coupling agents, such as are available from Dow Corning, may be used. Such coupling agents aid physical adhesion of the polymeric coating to the stent, but the coated articles still have relatively low shear resistance and longevity.
Although the above discussion has focused on the example of prosthetic heart valves, it is applicable to other implantable medical devices, such as catheters, cannulae, vascular grafts, pacemaker leads, defibrillator leads, needles, and orthopedic devices, among others.
Therefore, it is desirable to have implantable medical devices which do not stimulate adverse body reactions. It is also desirable to have implantable medical devices highly resistant to forces acting to shear two components apart. It is also desirable to provide a method for making implantable devices which exhibit both of the desirable traits given above.